1. The different colours of soil reflect processes of formation and evolution.
2. Generally, a dark brown or black soil is very fertile due to the largey amount of organic material it contains.
3. Grasslands where organic material from plant roots is incorporated into the soil usually are darkest.
4. Chernozem soils (from the Russian, meaning “black Earth”) in the western part of the American wheat belt and Ukranian wheat fields are among the world’s darkest and most fertile.
5. The amount of humus that has decomposed in the soil determines its organic content and creates the darker colour.
6. Red and yellow soils are a product of iron oxidation.
7. When organic material is weathered away by physical processes, mineral residues, especially iron oxides, are left behind.
8. The red and yellow soils are especially common in subtropical and tropical areas, where frequent and intense precipitation removes humus through leaching.